[Feature request] Support customize icon for back-to-top

[Mister-Hope]

Clear and concise description of the problem

The default icon, is emm....

Suggested solution

Procide a icon option for people to inejct svg strings or html string to customize back-to-top plugin.

Alternative

No response

Additional context

No response

[github-actions[bot]]

This issue is marked as stale because it has not had recent activity. Issues marked with stale will be closed if they have no activity within 3 days.